lyricsviewer small improvements

I made some changes to the code that I produced years ago for the plug-in lyricsviewer. I used this version for a few months without any problem.
The patch is divided into two files since the changes are independent, and you may wish to apply one of two (or even none, of course :D ).
The primary patch is dense mostly because I cleaned and reorganized a bit the code I wrote years ago, anyways there are also few "important" changes.

In summary:

Lyricsviewer Panel (lyricsviewer.patch):
- Cleaning and reorganization of the code
- Better integration with gtk system style
- improved management of concurrent requests

Lyricsviewer Preferences (lyricsviewer_pref.patch):
- Introduced the ability to reset the font used for lyrics according to the gtk system style